The present invention relates to a process for producing multilayer printed circuit board suited for wire bonding.
With a steady trend toward smaller size, higher performance and functional diversification of electronic appliances, the necessity for higher wiring density of multilayer printed circuit boards has intensified. In order to answer to this request, various attempts and improvements have been made, such as reduction of interlayer thickness, formation of fine wiring and diminishment of diameter of the interlaminar connecting holes. It has been also proposed and practiced to form interstitial via holes (IVH), which connect the adjoining interlaminar conductors alone, and beried via holes (BVH), and attempts are being made to further reduce the diameter of these IVH and BVH.
There are known two types of multilayer circuit boards: a type in which a plurality of circuit layers and interposed insulating layers are integrally laminated by hot pressing and electrically connected by forming holes at the necessary parts; and a built-up layer type in which circuit layers and insulating layers are formed alternately to constitute a built-up structure with the holes being formed at the necessary locations.
In a typical example of said built-up multilayer circuit board, plated through-holes in the interlayer circuit board having said through-holes and interlayer circuits therein are filled with a thermosetting resin by a proper means such as silk screen printing so that the holes will be perfectly closed, then the resin is cured by heating, and after removing the resin which has run out from the holes by suitable means such as polishing, a thermosetting resin is applied on the board and cured by heating to form an insulating layer, with the pertinent portions of this insulating layer being removed to provide the holes for interlaminar connection. Then the inner walls of these holes are metallized by plating or other means and the circuit conductors are formed on the insulating layers, followed by formation of the circuits thereon. By conducting the above operation on said interlayer cirucit board, it is possible to form another lamination of insulating layer and circuit layer, and by repeating this operation, there can be formed the desired multilayer circuits.
Such a built-up type multilayer circuit board tends to suffer a reduction of strength for the following reason. In the ordinary circuit boards, since a prepreg comprising a glass cloth impregnated with an epoxy resin or the like is used, the strength of the insulating layers can be maintained by incorporating a reinforcing material in the glass cloth, but in the case of the built-up multilayer circuit boards, no such reinforcement is used for facilitaing formation of the insulating layers by silk screen printing or formation of the holes for interlaminar connections. When attention is given only to the essential function of a circuit board, it seems that the above problem of low strength can be easily overcome by such means as adopting the above circuit board structure only at the sections where no high strength is required or providing a separate reinforcing sheet. Actually, however, it is often required to mount semiconductor devices with narrow terminal intervals or to mount the semiconductors themselves on the circuit board which is required to have a high wiring density, so that it is necessary to provide a base that guarantees a good balance of the semiconductors with the mechanical properties of the board and secure wire bonding used for direct mounting of the semiconductor parts.